In general, polypropylene resins having broad molecular weight distributions (of which polydispersity indexes as measured by a rheological method are typically larger than 4.0) exhibit better performance, because high molecular weight fraction of the resins imparts better mechanical strength, creep resistance, etc. to the resins, while low molecular weight fraction of the resins imparts excellent processability to the resins. Therefore, in some applications of high-performance polypropylene resins, such as hot-water pipes, BOPP films, etc., propylene polymers having broad molecular weight distributions are more competitive in comparison with propylene polymers having narrow molecular weight distributions.
In general, polypropylenes produced by using known high-activity Ziegler-Natta catalysts have narrower molecular weight distributions, with their polydispersity indexes (PI values) as measured by a rheological method being typically less than 4. Thus, multi-stage polymerization processes are mostly utilized to broaden the molecular weight distribution of polymers in the art, wherein the individual polymerization stages produce polymers having different molecular weight so that final polymers have broad molecular weight distribution (MWD). In each polymerization stage, the molecular weight of the polymers can be controlled by using a molecular weight control agent, such as hydrogen gas, or by altering polymerization temperature.
Such typical multi-stage polymerization processes comprise generally two or more stages of polymerization, wherein a first stage of polymerization is homopolymerization of propylene or copolymerization of propylene and an α-olefin carried out in the presence of a high-activity, highly-stereoselective Ziegler-Natta catalyst and a less amount of hydrogen, to provide a propylene homopolymer or copolymer having larger molecular weight, and a second stage of polymerization is homopolymerization of propylene or copolymerization of propylene and an α-olefin carried out, in the same reaction zone or in a different reaction zone, in the presence of the resulting polymer from the first stage of polymerization and a larger amount of hydrogen, to provide a propylene homopolymer or copolymer having less molecular weight.
It is generally accepted in the art that existing Ziegler-Natta catalysts are multi-site catalysts, in which those active sites having good hydrogen response have poor stereoselectivity, while those active sites having bad hydrogen response have good stereoselectivity. Because of this inherent characteristic of the Ziegler-Natta catalysts, isotacticities of fractions having different molecular weight of propylene polymers prepared through one-stage polymerization processes or traditional multi-stage polymerization processes (in which no means is used to adjust isotacticities of polymers produced in different polymerization stages) will be contrary to the requirements of high-performance materials, that is, low molecular weight fractions of the polymers have low isotacticities, while high molecular weight fractions of the polymers have high isotacticities. Such polymers may have many defects in the practical applications. For example, the fractions having low molecular weight and low isotacticities tend to migrate out from the interior of the materials during processing and during long-term use of articles, and thus adversely affect the performance and use of the articles. And the fractions having high molecular weight and high isotacticities tend to form thick lamellar crystal in the materials, and this is disadvantageous for some applications of propylene polymers. For example, when such resins are used to high-speed produce BOPP films, film breaking phenomenon occurs likely.
Thus, the current Ziegler-Natta catalyst-based processes for preparing propylene polymers having broad molecular weight distributions will cause the formation of a large amount of low molecular weight, low isotacticity fraction and a large amount of high molecular weight, high isotacticity fraction while broadening molecular weight distribution of the polypropylenes, so that it is impossible to obtain performance-optimized propylene polymers.